Orthogonal frequency division multiplexed (OFDM) communications systems have been developed to address problems in high data rate communications systems, such as multipath interference. In an OFDM system, a transmitter module receives an incoming data stream and modulates the data on orthogonal frequency domain sub-carriers. The modulated sub-carriers are then sent as an OFDM symbol to a receiver. In many OFDM systems a cyclic prefix (CP) is added to the OFDM symbol in the transmitter, typically by inserting a repeat of the end of the symbol in a guard interval at the front of the symbol. By dividing the incoming data stream among multiple sub-carriers, the data rate and thus the bandwidth of these individual sub-carriers is decreased relative to the bandwidth of the incoming data stream. The resulting increase in the duration of the data symbols associated with each sub-carrier can decrease the impact of multipath interference and associated inter-symbol interference (ISI).
One implementation advantage of OFDM systems is that Fast Fourier Transforms (FFTs) and Inverse Fast Fourier Transforms (IFFTs) can be used to simplify the demodulation and modulation processes, respectively. At an OFDM transmitter, incoming data signals are first demultiplexed into a plurality “N” of data sub-signals, each having a lower data rate than the incoming data. Each sub-signal is then translated in parallel into corresponding frequency domain symbols in a real or complex signal constellation (with example constellations using modulation such as QPSK or QAM). An IFFT bank can then be used to convert the frequency domain symbols into complex time-domain I and Q signals at a appropriate baseband or IF frequency, which are then combined to generate a transmit symbol. The transmit symbol can then be upconverted and transmitted at a desired transmit frequency.
At the receiver, an inverse process is applied to the incoming signal. In particular, the received time-domain signal may be quadrature mixed to generate I and Q signals, which are typically at baseband. The baseband signals are then sampled and digitized using analog-to-digital converters (“ADCs”). A forward FFT then operates upon a set of samples within a “window” of the composite received signal in order to convert the received signal back to a plurality of N parallel sub-carrier symbol streams, each of which is then converted to a corresponding binary data stream. These streams are then remultiplexed into a serial stream, which is an estimate of the incoming data stream provided to the transmitter.
However, the existence of Doppler and multipath conditions within the communication channel between the transmitter and receiver can impair the integrity of the transmit signal. Although the use of a cyclic prefix can partially mitigate the adverse effects of such degraded channel conditions, these conditions can cause shifting of the optimal location of the window used for FFT sampling. Failure to appropriately position this FFT window can reduce the quality of the signal produced by the receiver.